The efficiency of sub-THz to DC energy conversion of a silicon-based plasmonic detector was studied. The dependence of the signal at the detector output on the incident radiation power was measured. In the power linear region, the coefficient η was shown to grow with increasing power and to saturate in the sub-linear regime. The maximum achieved values of η were 0.4% for the radiation frequency of 97 GHz. The measurements were carried out both at room temperature and when the detector was cooled to liquid nitrogen temperature.